The present invention relates generally to remediation of pollutants in flue gases. More specific embodiments of the present invention relate to a wet gas scrubbing liquor which comprises an alkaline ferrous sulfide suspension for the treatment and removal of mercury from flue gases, and methods of using the wet gas scrubbing liquor to remove mercury from flue gases.
The emission of pollutants from coal-fired and oil-fired boilers is a major environmental concern. In particular, the emission of mercury into the atmosphere from these coal-fired boilers has garnered increased attention as a threat to human health and the environment since mercury, even at low concentrations, is a neurotoxin. Mercury is contained in varying concentrations in different coal supplies and therefore the total quantity of mercury emitted into the atmosphere from the combustion of coal can vary significantly between facilities.
During combustion of the coal, mercury is released in the form of elemental mercury (Hg0). As the combustion gases cool, a portion of the elemental mercury transforms to ionic or oxidized mercury (Hg2+) in the gas stream. As a result, there are three possible forms of mercury in gas streams that may be emitted to the atmosphere—elemental (Hg0), ionic or oxidized (Hg2+), or mercury that is bound onto particulates or fly ash also contained in the gas stream (Hg(p)).
The conversion of elemental mercury (Hg0) to the other forms of mercury is dependent upon several factors including, but not limited to, the cooling rate of the gas stream, the presence of halogens or sulfurous species (e.g. chlorines, bromines, SO32−), the amount and composition of fly ash, the presence of unburned carbon, and the removal efficiency of any installed air pollution control equipment. Considering the complex interaction of these various parameters, the form of mercury ultimately released to the atmosphere varies between 10% to 90%, 5% to 15%, and 10% to 90% of the total mercury for elemental, ionic, and particle bound species, respectively.
Mercury and other pollutants may be captured and removed from the gas stream by injection of a dry sorbent into the exhaust stream with subsequent collection in a particulate matter control device such as an electrostatic precipitator or a fabric filter. Systems based on dry sorbent technologies are collectively referred to as “dry scrubber” systems. Of the known dry sorbents for mercury removal, activated carbon and calcium-based sorbents have been the most actively studied and most widely used on a commercial scale.
Currently, the most commonly used sorbent in dry scrubber systems for mercury emission reduction is activated carbon that is injection in powder form into the flue gas stream of coal-fired and oil-fired plants. Although powdered activated carbon (PAC) is effective in capturing ionic or oxidized mercury species (Hg+2), powdered activated carbon is not as effective for removal of elemental mercury (Hg0) which can constitute a significant percentage of the mercury species in a flue gas, especially from facilities that burn subbituminous coals and lignite fuel sources. There have been efforts to enhance the trapping efficiency of elemental mercury (Hg0) in these systems by incorporating or impregnating the PAC with a bromine species.
Examples of other sorbents that have been used for mercury removal in dry scrubber systems include those disclosed in U.S. Patent Application Publication No. 2003/0103882 to Olson et al. and in U.S. Pat. No. 6,719,828 to Lovell et al. which teaches the use of sorbents such as clays having metal sulfides interlayered between clay layers to enhance mercury removal. Other patents disclose mercury capture by injection of dry sorbents which involve technologies that are based upon preparation of the sorbents by thinly layering a chemical compound onto or into a substrate. These types of sorbents use substrates disclosed in U.S. Pat. Nos. 7,790,830, 8,119,759 and 8,217,131 all to Edmiston that involve sol-gel derivatives; U.S. Pat. No. 8,088,283 to Pate, U.S. Pat. No. 8,142,664 to Krogue et al. and U.S. Pat. No. 8,197,687 to Krogue et al. that involve self-assembled monolayers on mesoporous supports; and U.S. Pat. No. 7,288,499 to Lovell et al. that involves the use of phyllosilicates, and a variety of other substrates. U.S. Pat. Nos. 7,575,629 and 7,704,920 both to Yang et al. disclose any metal salt that can release a metal ion when the salt contacts a sulfide salt forming a water insoluble metal sulfide on a substrate surface can be used to produce an effective dry sorbent for mercury removal.
Overall the production and use of these types of sorbents used in dry scrubbers for the removal of mercury from flue gases are complex and expensive.
Another type of scrubber system used to reduce the emission of mercury and other toxic gaseous pollutants into the environment is commonly referred to as a “wet scrubber”. In a wet scrubber system, polluted gases are brought into contact with a scrubbing liquid (scrubber liquor), either by spraying the gases with the liquid, by forcing the gases through a pool of liquid, or by some other contact method, so as to remove pollutants. The liquid compositions used in these wet scrubbers vary depending upon the pollutant targeted for removal. For example, in a wet flue gas desulfurization device (WFGD) used to remove acid gases such as sulfur dioxide (SO2), a sorbent slurry containing limestone (CaCO3), oxides or hydroxides of calcium or magnesium, or other mixtures are primarily used.
However, since elemental mercury is fairly insoluble in water (approximately 50 μg/L), elemental mercury is not effectively removed in wet scrubbers. Therefore, processes upstream of wet scrubbers that oxidize Hg0 in flue gases to Hg2+ will improve the effectiveness of overall mercury removal by the downstream wet scrubbers.
Since gas phase oxidation is kinetically limited, the need to oxidize any Hg0 to Hg2+, keeping any Hg2+ from being reduced back to Hg0 (sometimes referred to as “reconversion” or “re-emission”), and then finally capturing the Hg2+ in the wet scrubber process is essential.
Reactions with other ionic species that may be present in the gas stream and scrubber liquid also may have a significant impact on the ability of wet scrubber systems to remove Hg2+ and may effect the amount and rate of reconversion of Hg2+ to Hg0. In “Role of Sulfides in the Sequestration of Mercury by Wet Scrubbers,” by B. Ghorishi et al. (Presented at the EPRI-DOE-EPA-AWMA Combined Power Plant Air Pollutant Control Mega Symposium Aug. 28-31, 2006 in Baltimore, Md.), the authors proposed that once Hg2+ dissolves and ionizes in solution, it may subject to reactions with other dissolved constituents in a scrubber slurry. In the case of impurities such as divalent iron (Fe2+), the authors theorized that the reduction of Hg2+ to Hg0 may occur by the following reaction:2Fe2++Hg2+→Hg0+2Fe3+  (1)
Further, in the presence of aqueous sulfide ions, ionic or oxidized mercury (Hg2+) precipitates as HgS and effectively sequesters the Hg2+ as an insoluble solid according to the following reaction:HS−+Hg2+HgS↓+H+  (2)
Since reactions (1) and (2) occur simultaneously in wet scrubber systems, the amount of Hg0 subject to re-emission (reconversion) becomes a “race between the mercury reduction reactions” and the precipitation of Hg2+ as HgS. At Fe2+ concentrations of less than 1300 ppm in the scrubber liquor, all of the mercury is in the form of HgS and thus no Hg0 re-emission occurs. At higher Fe2+ concentrations (more than 1300 ppm) and higher pH (>4), any mercury present has a greater tendency to be re-emitted as Hg0.
Based on the above research, U.S. Pat. No. 6,284,199 to Downs et al., U.S. Pat. No. 6,503,470 to Nolan et al. and U.S. Pat. No. 6,855,859 to Nolan et al. each disclose methods to minimize the potential of this re-emission (reconversion) of the ionic or oxidized mercury (Hg2+) at the gas/liquid interface before it can be reduced by transition metals that may be present as impurities in the scrubber liquid.
Various means for supplying an aqueous source of sulfide ions to react with the oxidized mercury at the gas/liquid interface in the wet scrubber for the absorption and precipitation of ionized (oxidized) mercury include injecting a mixture of air and hydrogen sulfide as disclosed in U.S. Pat. No. 6,284,199 to Downs et al., and addition into the scrubber liquid of aqueous sulfide species such as sulfidic waste water, kraft caustic liquor, kraft carbonate liquor, potassium sulfide and sodium sulfide. To further inhibit the reconversion of Hg2+ to Hg0, U.S. Pat. No. 6,855,859 to Nolan et al. discloses an additional step whereby an oxidizing agent is first added to the scrubber liquid to convert any Hg0 present to Hg2+ in the flue gas prior to treatment of the Hg2+ with an aqueous sulfide ion.
In general the prior art related to mercury removal mechanisms by wet scrubbers systems rely upon absorption of ionic or oxidized mercury (Hg2+) by an aqueous sulfide ion. In these cases, absorption is a phenomenon whereby atoms, molecules, or ions that are present in the gas stream are absorbed (taken up) by the volume of the bulk (liquid) phase. On the other hand, “adsorption” is a physical phenomenon where atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere (bind) to another solid surface. The exact nature of the bonding by adsorption is dependent upon the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces), chemisorption (characteristic of covalent bonding), or some other type of electrostatic attraction. In other words, absorption is the process through which a substance, originally present in one phase, is removed from that phase by dissolution into another phase (typically a liquid), as opposed adsorption which is the accumulation of atoms, ions, or molecules from a bulk liquid or gas onto a solid surface.
The present invention overcomes the disadvantage of using sorbents in dry scrubbers which are primarily based on adsorption of mercury onto a sorbent and the disadvantage of wet scrubber systems which are based upon absorption of mercury by aqueous sulfide ions in the scrubber liquid to form an insoluble mercuric sulfide precipitant.